The invention is related to the field of cryptography and more specifically to cryptographically timestamping documents to prove their existence at a certain time
In many common situations people need to verify that a digital document (i.e. a document that is digitally stored in a computer system) existed on a certain date. That is, we may need to prove that no one has altered or revised the digital document since a certain date such as the alleged creation date or transmittal date of the document.
One method of providing such proof is known as electronic notarizing or timestamping. A one-way hash of the document is produced, and the hash is encrypted using a private key of the owner of the document to form a so called digital signature. The signature is sent to a digital notary or time stamper who combines the signature with a digital time (digital representation of the time and date) and encrypts the combination using the digital notary""s private key to form another digital signature. Then the notary sends his new digital signature to the author. The author can prove that the author""s signature for a report existed at the day of certification. Anyone with the notary""s public key can decrypt the notary""s signature and prove that the document was originally encrypted by someone who had access to the private key.
Notarizing digital documents is disclosed in U.S. Pat. No. 5,136,646. Notarizing by secure hardware in a system is disclosed in U.S. Pat. No. 5,001,752. Public key cryptography is disclosed xe2x80x9cNew Directions in Cryptographyxe2x80x9d by Diffie and Hellman in IEEE Transactions On Information Theory, Vol IT-22, November 1976, pp 644-654 and in U.S. Pat. Nos. 4,405,829 to Rivest and 4,868,877. One-way hashing is disclosed in xe2x80x9cCollision-Free Has Functions and Public Key Signature Schemesxe2x80x9d, Advances in Cryptologyxe2x80x94eurocrypt ""87, Springer-Verlag, LNCS, 1988, vol. 304, pp. 203-217.
The above citations are hereby incorporated in whole by reference.
It is an object of the invention to provide methods and apparatus for the authentication of revisions.
In the inventions disclosed herein an original document and a revised document are notarized so that relationship between the original document and revised document can be proved as well as the origination and the time of the revisions notarization.
In one embodiment of the invention the original document is notarized, then later the document is revised and the revision and its relationship to the original document is notarized. In another embodiment the original document and an automatically generated revision of the document is simultaneously notarized. This allows the authorship and generation time of automatically generated revisions such as a lossy compression of information to be proved.
Other alternatives and advantages of applicant""s inventions will be disclosed or become obvious to those skilled in the art by studying the detailed description below with reference to the following drawings which illustrate the elements of the appended claims of the inventions.